1. Field of the Invention
The present invention relates to a substrate with a zinc oxide layer (hereinafter often referred to as a ZnO-layered substrate) used, for example, as a component of a photovoltaic device such as a solar cell or the like, a method for forming the zinc oxide layer, a photovoltaic device, and a method for producing the photovoltaic device and, more particularly, to a substrate provided with a zinc oxide layer as a part of a reflecting layer for improving the long-wavelength sensitivity of the solar cell and a method for forming it.
2. Related Background Art
The applicants already suggested a combination of a metal layer with a transparent conductive layer made by sputtering, which was used as a reflecting layer of the photovoltaic device (solar cell). In this suggestion, sputter conditions of the metal layer for suppressing decrease of reflectance of the metal layer are detailed in order to obtain the reflecting layer with good reflection characteristics. The sputtering method, however, requires time and labor for production of a target even if inexpensive materials are used. Thus, the cost of the target is not low and the efficiency of utilization thereof is as low as approximately 20%. The material cost is, therefore, very high. Moreover, a sputter apparatus is a vacuum device and the apparatus is thus expensive. As a result, the depreciation cost is also high. These factors hinder providing inexpensive solar cells to solve environmental issues. The applicants also suggested a method for depositing two kinds of zinc oxide layers on a long rolled substrate (hereinafter referred to as xe2x80x9clong substratexe2x80x9d) by electrolytic deposition (electrodeposition) being excellent in the optical confinement effect and industrially very inexpensive. This method at the present time, however, does not allow the zinc oxide layers to be deposited directly on an aluminum layer which is inexpensive and which has high reflectance. Grounds thereof are that the surface of aluminum is modified into boehmite by a hot acid solution, thereby extremely degrading the reflection and that the boehmitemodified surface is of crumple-like structure whereby growth of zinc oxide is also of the crumple shape.
An object of the present invention is, therefore, to inexpensively provide a ZnO-layered substrate excellent in reflection and the optical confinement effect and useful as a substrate with a reflecting layer for a solar cell, solving the above problems.
The present invention provides a ZnO-layered substrate in which at least a zinc oxide layer is provided on a support substrate, wherein the zinc oxide layer comprises a zinc oxide layer having the c axis perpendicular to the support substrate and a zinc oxide layer having the c axis slantindicular to the support substrate in the order from the side of the support substrate. The present invention also provides a method for forming a zinc oxide layer, comprising a step of forming on a substrate a zinc oxide layer having the c axis perpendicular to the substrate by a sputtering method, and a step of forming a zinc oxide layer having the c axis slantindicular to the substrate on the zinc oxide layer having the c axis perpendicular to the substate, by an electrodeposition method.
Further, the present invention provides a photovoltaic device comprising a ZnO-layered substrate in which at least a zinc oxide layer is provided on a support substrate, and a semiconductor layer, wherein the zinc oxide layer comprises a zinc oxide layer having the c axis perpendicular to the support substrate and a zinc oxide layer having the c axis slantindicular to the support substrate in the order from the side of the support substrate.
In addition, the present invention provides a method for producing a photovoltaic device, comprising a step of producing a ZnO-layered substrate by forming on a support substrate a zinc oxide layer having the c axis perpendicular to the support substrate by a sputtering method and forming a zinc oxide layer having the c axis slantindicular to the support substrate on the zinc oxide layer having the c axis perpendicular to the support substrate, by an electrodeposition method, and a step of forming a semiconductor layer on the ZnO-layered substrate.
In the present invention, the substrate (support substrate) is preferably an electrically conductive substrate. The support substrate is preferably an SUS sheet and the SUS sheet preferably has a 2D-surface. It is also preferable to use an SUS substrate of a long roll form.
In the ZnO-layered substrate and the photovoltaic device of the present invention, a metal layer is preferably interposed between the support substrate and the zinc oxide layer. The thickness of the metal layer is preferably 1000 xc3x85 to 2500 xc3x85. The metal layer is preferably a metal aluminum layer. A temperature of the substrate in forming the metal aluminum layer by sputtering is preferably set at 100xc2x0 C. or less. It is preferable to interpose an aluminum oxide layer between the metal aluminum layer and the zinc oxide layer. The aluminum oxide layer is preferably one formed by oxidizing the metal aluminum layer by an oxygen plasma process. On that occasion, the power of the oxygen plasma in the oxygen plasma process is preferably so set that a total reflectance of light incident from the side of the zinc oxide layer having the c axis slantindicular to the substrate is not less than 60% and that electric resistance in a direction normal to the surface of the substrate is not more than 20 xcexa9/cm2.
Further, an average of inclination angles of crystal grains of a surface of the zinc oxide layer having the c axis slantindicular to the support substrate on an opposite side to the support substrate is preferably not less than 15xc2x0; surface roughness Ra of the opposite surface of the layer to the support substrate is preferably not more than 80 nm; the thickness of the layer is preferably not less than 5000 xc3x85. The zinc oxide layer having the c axis slantindicular to the substrate is preferably formed by electrodeposition using a zinc nitrate solution having a concentration of not less than 0.15 mol/l.
The thickness of the zinc oxide layer having the c axis perpendicular to the substrate is preferably 1500 xc3x85 to 2500 xc3x85. The temperature of the substrate in forming the layer by sputtering is preferably set at 380xc2x0 C. or less. The temperature of the substrate in forming the zinc oxide layer having the c axis perpendicular to the substrate is preferably so set that an average grain size of crystal grains observed in the surface of the zinc oxide layer having the c axis slantindicular to the substrate on the opposite side to the substrate is not more than 2 xcexcm and that a scattering reflectance of light incident from the side of the zinc oxide layer having the c axis slantindicular to the substrate is not less than 20% at 800 nm. The temperature of the substrate in forming the zinc oxide layer having the c axis perpendicular to the substrate is preferably so set that surface roughness Ra of the surface of the zinc oxide layer having the c axis slantindicular to the substrate on the opposite side to the substrate is not more than 80 nm and that an average of inclination angles of crystal grains in the surface is not less than 15xc2x0.
Further, a total reflectance of light incident from the side of the zinc oxide layer into the ZnO-layered substrate of the present invention is preferably not less than 60% at 800 mn and a scattering reflectance is preferably not less than 20% at 800 nm.